Particle–Seawater Interaction of Neodymium in the North Atlantic

Torben Stichel,Tina Van De Flierdt,Yves Plancherel,Myriam Lambelet,Michiel Rutgers Van Der Loeff,Walter Geibert,Sven Kretschmer

doi:10.1021/acsearthspacechem.0c00034

Abstract

Dissolved neodymium (Nd) isotopes (expressed as εNd) have been widely used as a water mass tracer in paleoceanography. However, one aspect of the modern biogeochemical cycle of Nd that has been sparsely investigated is the interplay between dissolved and particulate phases in seawater. We here present the first regional data set on particulate Nd isotope compositions (εNdp) and concentrations ([Nd]p) from five stations in the western North Atlantic Ocean along the GEOTRACES GA02 transect, in conjunction with previously published dissolved Nd isotope compositions (εNdd) and concentrations ([Nd]d)1. Key observations and interpretations from our new particulate data set include the following: (1) Low fractional contributions of [Nd]p to the total Nd inventory per volume unit of seawater (∼5%), with significant increases of up to 45% in benthic boundary layers. (2) Increasing Nd concentrations in suspended particulate matter ([Nd]SPM) and fractions of lithogenic material with water depth, suggesting the removal of Nd poor phases. (3) Different provenances of particulates in the subpolar and subtropical gyres as evidenced by their Nd isotope fingerprints reaching from εNdp ≈ −20 near the Labrador Basin (old continental crust), over εNdp ≈ −4 between Iceland and Greenland (young mafic provenance), to values of εNdp ≈ −13 in the subtropics (similar to African dust signal). (4) Vertical heterogeneity of εNdp as well as large deviations from ambient seawater values in the subpolar gyre indicate advection of lithogenic particles in this area. (5) Vertically homogeneous εNdp values in the subtropical gyre, indistinguishable from εNdd values, are indicative of predominance of vertical particulate supply. The process of reversible scavenging only seems to influence particulate signatures below 3 km. Overall, we do not find evidence on enhanced particle dissolution, often invoked to explain the observed increase in dissolved Nd in the North Atlantic.

Highlights

The region is a key area for ocean circulation: North Atlantic Deep Water (NADW) is formed in the subpolar gyre and is constituted by Upper and Classical Labrador Seawater (ULSW, CLSW) from the Labrador Basin and overflow waters from the Northeastern Atlantic Ocean.[31,32]
We presented the first regional data set of particulate and dissolved Nd concentrations and isotope compositions from the subpolar and subtropical North Atlantic
Particulate Nd concentrations ([Nd]p) are rather invariant throughout the water column as opposed to the strongly elevated values found near the seafloor for all but one station

Summary

Introduction

The region is a key area for ocean circulation: North Atlantic Deep Water (NADW) is formed in the subpolar gyre and is constituted by Upper and Classical Labrador Seawater (ULSW, CLSW) from the Labrador Basin and overflow waters from the Northeastern Atlantic Ocean.[31,32] Denmark Strait Overflow Water (DSOW) and Iceland Scotland Overflow Water (ISOW) mix within the Irminger Basin.[33] The mixture of the deep parts of LSW and the overflow waters is what forms Irminger Sea water, often referred to as Lower NADW,[34] whereas Upper NADW consists largely of ULSW and CLSW.[35] The resulting NADW flows southward along the western margin of the Atlantic Ocean to form the Deep Western Boundary Current, (e.g., ref 36) This deep southward current is accompanied by the counter flow of the Gulf Stream (GS) at the surface and Southern Ocean waters at the bottom.[31,37]

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Discussion

Conclusion